Alistair and Bryan builds a toy car in 6 hours.
Bryan and Chester builds a toy car in 3 hours.
Chester and Alistair builds a toy car in 7 hours.
How long does it take for Alistair, Bryan and Chester to build a toy car together?
Fraction of the toy car built by Alistair and Bryan in 1 hour = ¹₆ 
Fraction of the toy car built by Bryan and Chester in 1 hour = ¹₃ 
Fraction of the toy car built by Chester and Alistair in 1 hour = ¹₇ 

Total fraction of the toy car built by Alistair and Bryan and Chester in 1 hour
= (¹₆ + ¹₃ + ¹₇) ÷ 2
= (⁷₄₂ + ¹⁴₄₂ + ⁶₄₂) ÷ 2
= ²⁷₄₂ ÷ 2
= ⁹₁₄ x ¹₂
= ⁹₂₈ 

Time taken for Alistair, Bryan and Chester to build a toy car together
= 1 ÷ ⁹₂₈
= 1 x ²⁸₉
= 3¹₉ h

Answer(s): 3¹₉ h or 3¹₉ hours